Average value (±SD) of the parameters derived from esophageal pressure measurements under different settings of BIPAP (biphasic positive airway pressure) mechanical ventilation: a) controlled ventilation (with animal paralyzed using atracurium bromide); b) assisted ventilation (allowing unsupported spontaneous breaths), considering only the controlled breaths; c) and d) assisted ventilation, considering only the unsupported spontaneous breaths.

<p>PEEP: positive end-expiratory pressure (cmH₂O); ΔP: driving pressure (cmH₂O); SUP, LAT: supine or right lateral decubitus; E_L, E_cw: lung and chest wall elastance; ΔP_trans: inspiratory change in transpulmonary pressure; P_{trans,end-exp}: end-expiratory value of transpulmonary pressure; PTP: pressure-time product; WOB_i: inspiratory work of breathing. The p-value of the effect of measurement method, position (SUP/LAT), PEEP and ΔP was computed using a general linear model approach (n.s. corresponds to p-value>0.05). &: significant (p<0.05) difference between indexes estimated using the air-filled and liquid-filled catheter, using a paired t-test. Similar notation for the comparisons of the balloon catheter with the air-filled ($) and liquid-filled (*) catheter.</p

Bland-Altman plots comparing average parameters estimated using the balloon catheter measurement with those obtained using the air- or liquid-filled catheter, under different settings of BIPAP (biphasic positive airway pressure) mechanical ventilation.

<p>E_L, E_cw: lung and chest wall elastance; ΔP_trans: inspiratory change in transpulmonary pressure; P_{trans,end-exp}: end-expiratory value of transpulmonary pressure; PTP: pressure-time product; WOB_i: inspiratory work of breathing. The horizontal lines represent the mean (solid line) ± SD (dashed lines) of the difference between methods.</p

Illustration of the combination of catheters used for the experiment.

<p>The catheters were permanently fastened together, with the tip of the air- and liquid-filled catheters positioned 2 cm further down the esophagus than the tip of the balloon. Additional 4 holes were created in the liquid- and air-filled catheters to minimize the possibility of obstruction, located equidistantly along the circumference of the catheter at approximately 1 cm from the tip.</p

Bias (precision) of parameters estimated using the liquid- or air-filled catheters compared to those obtained with the balloon catheter, in different conditions.

<p>BIPAP: biphasic positive airway pressure; contr MV: controlled mechanical ventilation (with animal paralyzed using atracurium bromide); contr BIPAP: assisted ventilation (allowing unsupported spontaneous breaths), considering only the controlled breaths; spont BIPAP: assisted ventilation, considering only the unsupported spontaneous breaths. E_L, E_cw: lung and chest wall elastance; ΔP_trans: inspiratory change in transpulmonary pressure; P_{trans,end-exp}: end-expiratory value of transpulmonary pressure; PTP: pressure-time product; WOB_i: inspiratory work of breathing. Significance of t-test:</p><p>*p<0.05;</p><p>**p<0.01;</p><p>***p<0.001.</p><p>Bias (precision) of parameters estimated using the liquid- or air-filled catheters compared to those obtained with the balloon catheter, in different conditions.</p

Representative esophageal pressure measurement tracings during a) controlled mechanical ventilation and b) biphasic positive airway pressure (BIPAP) ventilation with spontaneous breathing (note: offsets between the tracings are just for pictorial representation).

<p>The balloon catheter tracings show large oscillations that are coherent with the heart-beats occurrences in the EKG (the dotted vertical lines represent the occurrence of the R-peaks of the EKG, i.e. ventricular contraction). To the right side of the tracings the power spectra of each esophageal pressure signal is shown, disclosing that the largest amount of the power is concentrated at frequencies compatible with the respiratory spectrum (dashed line represents the average respiratory rate). Interestingly, only the balloon catheter presents a peak of considerable power at frequencies compatible with the EKG spectrum (the dash-dot lines represent the average heart rate). The pressure-time product (PTP) is graphically represented by the gray areas in panel b): in the balloon tracings it is evident that cardiogenic noise affects the shape of the area, modifying the estimated value of PTP.</p

Correlation of Lung Collapse and Gas Exchange - A Computer Tomographic Study in Sheep and Pigs with Atelectasis in Otherwise Normal Lungs

<div><p>Background</p><p>Atelectasis can provoke pulmonary and non-pulmonary complications after general anaesthesia. Unfortunately, there is no instrument to estimate atelectasis and prompt changes of mechanical ventilation during general anaesthesia. Although arterial partial pressure of oxygen (PaO₂) and intrapulmonary shunt have both been suggested to correlate with atelectasis, studies yielded inconsistent results. Therefore, we investigated these correlations.</p><p>Methods</p><p>Shunt, PaO₂ and atelectasis were measured in 11 sheep and 23 pigs with otherwise normal lungs. In pigs, contrasting measurements were available 12 hours after induction of acute respiratory distress syndrome (ARDS). Atelectasis was calculated by computed tomography relative to total lung mass (M_total). We logarithmically transformed PaO₂ (lnPaO₂) to linearize its relationships with shunt and atelectasis. Data are given as median (interquartile range).</p><p>Results</p><p>M_total was 768 (715–884) g in sheep and 543 (503–583) g in pigs. Atelectasis was 26 (16–47) % in sheep and 18 (13–23) % in pigs. PaO₂ (FiO₂ = 1.0) was 242 (106–414) mmHg in sheep and 480 (437–514) mmHg in pigs. Shunt was 39 (29–51) % in sheep and 15 (11–20) % in pigs. Atelectasis correlated closely with lnPaO₂ (R² = 0.78) and shunt (R² = 0.79) in sheep (P-values<0.0001). The correlation of atelectasis with lnPaO₂ (R² = 0.63) and shunt (R² = 0.34) was weaker in pigs, but R² increased to 0.71 for lnPaO₂ and 0.72 for shunt 12 hours after induction of ARDS. In both, sheep and pigs, changes in atelectasis correlated strongly with corresponding changes in lnPaO₂ and shunt.</p><p>Discussion and Conclusion</p><p>In lung-healthy sheep, atelectasis correlates closely with lnPaO₂ and shunt, when blood gases are measured during ventilation with pure oxygen. In lung-healthy pigs, these correlations were significantly weaker, likely because pigs have stronger hypoxic pulmonary vasoconstriction (HPV) than sheep and humans. Nevertheless, correlations improved also in pigs after blunting of HPV during ARDS. In humans, the observed relationships may aid in assessing anaesthesia-related atelectasis.</p></div

Time course of respiratory system elastance and slope.

<p><b>A.</b> Respiratory system elastance (Ers) throughout ventilation protocols. Red points are minute-averaged values of Ers and its fitted line (blue line). <b>B.</b> Slope of Ers calculated by angular coefficient of the groups: VCV_E (volume-controlled mode and PEEP_minErs; n = 6), VCV_E+2 (volume-controlled mode and PEEP_minErs plus 2 cmH₂O; n = 6), VCV_E-2 (volume-controlled mode and PEEP_minErs minus 2 cmH₂O; n = 6), VV_E (variable ventilation and PEEP_minErs; n = 6), VV_E+2 (variable ventilation and PEEP_minErs plus 2 cmH₂O; n = 5), VV_E-2 (variable ventilation and PEEP_minErs minus 2 cmH₂O; n = 6). Effect of ventilator mode, PEEP and interaction were tested with two-way ANOVA followed by Tukey post-hoc. Time effect was tested by generalized linear model. Statistical significance was accepted at p<0.05. *p<0.05.</p

Lung cytokines.

<p>Lung tissue concentration of TNF-α (A), IL-6 (B), CINC-1(C) and IL-1β(D) in the groups: VCV_E (volume-controlled mode and PEEP_minErs; n = 6), VCV_E+2 (volume-controlled mode and PEEP_minErs plus 2 cmH₂O; n = 6), VCV_E-2 (volume-controlled mode and PEEP_minErs minus 2 cmH₂O; n = 6), VV_E (variable ventilation and PEEP_minErs; n = 6), VV_E+2 (variable ventilation and PEEP_minErs plus 2 cmH₂O; n = 5), VV_E-2 (variable ventilation and PEEP_minErs minus 2 cmH₂O; n = 6). Red lines represent mean values. Ventilatory mode and PEEP effects were tested with two-way ANOVA followed by Tukey post hoc. T-test was used to detect differences between ventilated and non-ventilated groups and then correction by multiple comparison. Statistical significance was accepted at p<0.05. *p<0.05.</p

Ventilatory and hemodynamic variables.

<p>Values are mean ± SD from initial 5 minutes of ventilation. PEEP values are given as range (minimum and maximum). Definition of abbreviations: VCV_E = volume-controlled mode and PEEP_minErs; VCV_E+2 = volume-controlled mode and PEEP_minErs plus 2 cmH₂O; VCV_E-2 = volume-controlled mode and PEEP_minErs minus 2 cmH₂O; VV_E = variable ventilation and PEEP_minErs; VV_E+2 = variable ventilation and PEEP_minErs plus 2 cmH₂O; VV_E-2 = variable ventilation and PEEP_minErs minus 2 cmH₂O; BW = body weight; Ppeak = peak airway pressure; Pmean = mean airway pressure; PEEP = positive end-expiratory pressure; V_T = tidal volume; RR = respiratory rate; CVVT = tidal volume coefficient of variation; MAP = mean arterial pressure; HR = heart rate. Differences among groups were tested with two-way ANOVA. Statistical significance was accepted at p<0.05. **p<0.0001, ns: non-significant.</p><p>Ventilatory and hemodynamic variables.</p

Gas exchange.

<p>Values are expressed by mean ± SD. Definition of abbreviation: VCV_E: volume-controlled mode and PEEP_minEr; VCV_E+2: volume-controlled mode and PEEP_minErs plus 2 cmH₂O; VCV_E-2: volume-controlled mode and PEEP_minErs minus 2 cmH₂O; VV_E: variable ventilation and PEEP_minErs; VV_E+2: variable ventilation and PEEP_minErs plus 2 cmH₂O; VV_E-2: variable ventilation and PEEP_minErs minus 2 cmH₂O. Differences between groups were tested with Two-way ANOVA. Statistical significance was accepted at p<0.05.</p><p>Gas exchange.</p